Lyme disease is the most prevalent tick-borne disease in the USA and affects the heart, joints, skin, musculoskeletal and nervous systems. Recently, the reported cases in the USA have increased to e20,000 per year. The causative agent, Borrelia burgdorferi, often persists in the face of a strong immune response and can result in incapacitating chronic disease manifestations, such as arthritis or neuroborreliosis. My long-term goal is to describe the host-bacterial interactions essential for establishment of the bacterial infection and consequently Lyme disease. Colonization of various tissues by many microbial pathogens requires attachment to cells or extracellular matrix (ECM). Glycosaminoglycans (GAGs) are carbohydrate units of proteoglycans, which are ubiquitously expressed ECM components on mammalian cells. Our extensive studies show that GAG-binding is a major adherence mechanism of B. burgdorferi. Indeed, we have shown that B. burgdorferi produces several GAG- binding adhesins, including, (i) DbpA and DbpB, (ii) Borrelia GAG-binding Protein (Bgp), and (iii) BBK32. Consistent with the multiplicity of GAG-binding adhesins, mutants lacking one or two documented adhesins of Lyme spirochetes retain some degree of infectivity. B. burgdorferi dbpA-dbpB double deletion mutants, generated in a variety of strain backgrounds, our B. burgdorferi bgp mutants, and bbk32 mutants remain infectious in immunocompetent mice. At least first three of these exhibit colonization defect. Bgp also exhibits 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase activity. However, it is not known if the colonization defect of the bgp mutant is related to its GAG-binding activity, its nucleosidase activity, or both. To determine if the defect in colonization by a bgp mutant is related to its GAG-binding, its enzymatic activity, or both, and whether Bgp displays partial functional redundancy with other GAG-binding adhesins, the following aims will be pursued. (1). Delineate the GAG-binding and MTA/SAH nucleosidase activities of Bgp. (2). Define the activity of Bgp required for colonization during infection. (3). Assess the functional redundancy of Bgp with other GAG-binding adhesins during colonization. Significance. We first recognized Bgp as a multifunctional surface protein, observed its role in tissue colonization during mouse infection, and determined it to be a target of novel drugs. This study will genetically delineate two activities of Bgp to assess if one or both activities are required during infection. We will also determine if defect in Bgp and DbpA-DbpB production is additive or each contribute to the tissue- specific colonization by B. burgdorferi. Such an understanding of the spirochete-GAG interactions involving Bgp, DbpA and DbpB, on Lyme pathogenesis and of the role of Bgp as a metabolic enzyme could help design better diagnostic tests and facilitate the development of novel treatment strategies, especially for the later stages of Lyme disease.